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Abstract

The clay mineralogy of a pink granophyre of early Permian age, overlain locally by outcrops of Tertiary arkose, has been examined using thin section petrography, X-ray powder diffraction (XRD), electron probe microanalysis, measurement of exchangeable cations, and methylene blue adsorption. The rock consists of quartz, potassium feldspar, and plagioclase feldspar as major, and mica and chlorite as minor components. The feldspars show an increasing amount of alteration in slightly weathered and moderately weathered rocks. XRD showed that chlorite had partly altered to smectite in the slightly weathered rock, probably through a vermiculite intermediate phase. In the moderately weathered rock, chlorite is completely altered to smectite. The increase of methylene blue adsorption, total exchangeable cations, and the Mg/Ca ratio with weathering supports a progressive alteration of chlorite to smectite. Electron microprobe analyses of the clay phases in these rocks were variable, but indicated a considerable depletion in Mg (maximum MgO = 2.70%).

Kaolinite occurs mainly in the completely weathered rock at the surface (0–0.6-m depth), in highly to moderately weathered rock to a depth of 1 m, in a horizontal joint at a depth of 3.5 m, and in some, but not all, deeper samples of the highly to moderately weathered rock. Although plagioclase is presumeably the source of most of the kaolinite in these rocks, some of the kaolinite in samples from 0–1-m depth could have been contributed by weathered arkose that once covered the granophyre.

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